The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting a polishing endpoint during a chemical mechanical polishing operation.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, planarization is often needed to remove a filler layer until an underlying stop layer is exposed, or to create a layer with a defined thickness.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. Conventionally, the exposed surface of the substrate is placed against a rotating polishing pad, although a linear belt or other polishing surface can be used. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad (also, some polishing processes use a “nonabrasive” process).
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness or whether an underlying layer has been exposed. If an excessive amount of material is removed (overpolishing), the substrate is rendered unusable. If, on the other hand, if an insufficient amount of material is removed (underpolishing), the substrate must be reloaded into the CMP apparatus for further processing. This is a time-consuming procedure that reduces the throughput of the CMP apparatus.
The polishing rate is sensitive to the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate, as well as the initial substrate topography. In addition, there can be variations in the thickness of the layers in the substrate layers. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
Various methods are used to monitor and control the CMP planarity and layer thickness during polishing. For example, the substrate thickness can be monitored in-situ by an optical sensor, such as an interferometer. Alternatively, exposure of an underlying layer and the associated change in reflectivity of the substrate can be detected a reflectometer. In addition, various method are used to measure the layer thickness before or after thickness (e.g., in an in-line metrology station). For example, a spectrometer, such as the NovaScan 210, manufactured by the Nova Corporation of Israel, can be used an in-line metrology device.
Although these techniques are satisfactory, there is still room for improvement in the accuracy of the determination of the polishing endpoint.